Composite electrodes for solid state devices

ABSTRACT

A solid state ionic device includes a dense electrolyte sandwiched between two porous electrodes. In one embodiment, the device is anode supported and the cathode is formed of a porous three-dimensional solid phase structure having an electrocatalytic phase of a plurality of electrocatalytic particles and an ionic conducting phase of a plurality of ionic conductor particles. The mean or median size of the electrocatalytic particles is larger than the mean or median size of the ionic conductor particles. The device may further include a long range electronic conducting layer of lanthanum cobaltate or other electronically conducting material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/684,660 filed on Oct. 10, 2000 now U.S. Pat. No. 6,420,064,which claims the priority benefit of U.S. Provisional Application Nos.60/158,124 filed on Oct. 8, 1999 (Solid Oxide Fuel Cell CompositeElectrode), and 60/231,542, filed Sep. 11, 2000 (Improved CompositeElectrodes For Solid State Devices), which applications are incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to composite electrodes and functionallayers for electrode reactions for use with solid-state ionic devices,and solid oxide fuel cells in particular.

BACKGROUND OF THE INVENTION

The following references are referred to herein by their numericalreference and the contents of each is incorporated herein by reference.

1. Erning, J. W., Hauber, T., Stimming, U. Wippermann, K., Catalysis ofthe electrochemical processes on solid oxide fuel cell cathodes, Journalof Power Sources 61 (1996) 205-211.

2. M. Watanabe, H. Uchida, M. Shibata, N. Mochizuki and K. Amikura, Highperformance catalyzed—reaction layer for medium temperature operatingsolid oxide fuel cells, J. Electrochem. Soc., vol. 141, (1994) 342-346.

3. Sahibzada, M., Benson, S. J., Rudkin, R. A., Kilner, J. A.,Pd-promoted La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ cathodes. Solid StateIonics 113-115 (1998) 285-290.

4. M. M. Murphy, J. Van herle, A. J. McEvoy, K. Ravindranathan Thampi,Electroless deposition of electrodes in solid oxide fuel cells, J.Electrochem. Soc., vol. 141 (1994) 30 L94-96.

5. Uchida et al. Shin-ichi Arisaka and Masahiro Watanabe, Paper B-IN-05at 121^(st) International Conference on Solid State Ionics (1999)154-155.

Background

Solid state ionic devices typically consist of a fully dense electrolytesandwiched between thin electrode layers. It is known that the principallosses in most solid state ionic devices occur in the electrodes or theelectrode/electrolyte interfaces. Therefore, minimization of theselosses is critical to efficient operation of these devices.

Solid oxide fuel cells (SOFC) are theoretically very efficient energyconversion devices that have the potential of becoming a commercialproduct for numerous uses. A SOFC is a solid electrochemical cell whichconsists of a solid electrolyte impervious to gases, sandwiched betweena porous cathode and a porous anode. Oxygen gas is transported throughthe cathode to its interface with the electrolyte where it is reduced tooxygen ions, which migrate through the electrolyte to the anode. At theanode, the ionic oxygen reacts with fuels such as hydrogen or methaneand release electrons. The electrons travel back to the cathode throughan external circuit to generate electric power.

The construction of conventional SOFC electrodes are well known.Electrodes are often applied as composites of an electron conductingmaterial and an ion conducting material. For instance, an anode mayconsist of electronic conducting nickel (Ni) and ionic conducting yttriastabilized zirconia (YSZ) while the cathode may consist of a perovskitesuch as La_(1−x)Sr_(x)MnO_(3−δ) (LSM) as the electron conductingmaterial and YSZ as the ion conductor.

Conventional SOFCs exhibit high performance at operating temperatures of1000° C. However, such high temperature operation has disadvantages suchas physical or chemical degradation of the construction materials.Therefore, it is desirable to reduce the operating temperature of a SOFCstack to a medium temperature of about 700° C. However, at such mediumtemperatures, electrode reaction rates decrease significantly. Prior artefforts to increase electrode reactivity at lower temperatures havefocussed on optimizing the electrode microstructure and by introducingcatalytic materials into the electrode structure.

It is well known to provide an activated surface on the fuel cellelectrodes by means of a catalyst to aid the electrochemical process.Nickel is commonly used as a catalyst on the anode side for oxidation offuel. On the cathode side, ceramic cathode materials typically used inSOFCs, such as perovskites have a high activation energy for oxygenreduction. Therefore, the activation energy may be reduced for theoxygen reduction reaction by adding noble metals such as Au, Ag, Pt, Pd,Ir, Ru and other metals or alloys of the Pt group. Erning et al. [1]reported that addition of highly dispersed noble metals (<=0.1 mg/cm²)lowers the activation energy of the oxygen reduction reaction at thecathode of an SOFC. M. Watanabe [2] also found that the anodicpolarization resistance and its activation energy were greatly decreasedby loading only a small amount of catalyst such as Ru, Rh, and Pt onto asamaria-doped ceria (SDC) anode. A large depolarizing effect was alsoobserved with a Pt-catalyzed LSM cathode, especially at high currentdensities. Sahibzada et al. [3] has recently reported that LSCFelectrodes which were impregnated with small amounts of Pd resulted in3-4 times lower cathodic impedance in the temperature range 400 to 750°C. The overall cell resistance decreased 15% at 6500° C. and 40% at 550°C.

For economic reasons, noble metal catalysts are applied in very smallamounts to catalyze the electrochemical process at electrodes. Thecatalysts are conventionally impregnated in the pores of the electrodeby a filtration or a chemical process. The impregnation process isfrequently followed by a binding process where a binder is superimposedon the deposited particles to provide a secure and durable attachment ofthe coating with the base material. U.S. Pat. Nos. 3,097,115; 3,097,974;3,171,757 and 3,309,231 disclose such conventional impregnatingprocesses for porous electrodes.

The catalysts may also be applied by common electroless depositiontechniques for Ni, Pd and Ag [4] and replacement plating, as disclosedin U.S. Pat. No. 3,787,244. In this process, an acidic plating solutioncontaining a salt of a noble metal catalyst is forced through the poresof a nickel electrode substrate and the noble metal ions from thedissolved salt replace a thin layer of the nickel surface within thepores.

It is known [1] to form highly dispersed catalyst layers with an amountof less than 0.1 mg/cm² from aqueous solutions of Pt, Pd, Ir or Rusalts. A few drops of these solutions were applied onto the electrolytesurface. After drying, the salts were either reduced to metal form byheating under hydrogen (Pt and Pd) or oxidized by heating under air (Irand Ru), Most recently, Uchida et al. [5] applied nanometer-sized noblemetal catalysts to both anode and cathode resulting in appreciably loweroverpotential ohmic resistance.

Singheiser (EP 424813) discloses an intermetallic compound layer (0.5-5μm) contains 2-70 wt. % of a noble metal such as Pt, Ag or Pd which canbe used between electrolyte and electrodes, or to connect electricallytwo fuel-cells. It is claimed that the fuel cell can be operated at alower temperature due to higher electrode conductivity.

Because of the cost of noble metals, the application of noble metals inSOFC electrodes so far are mainly limited to its catalytic abilities.All recent efforts have been to add very fine particles of the catalystin order to maximize the three phase boundary of the catalyst, the gasphase and the electrolyte. The catalyst is either applied as a very thinlayer at the electrolyte/electrode boundary or is widely dispersedthroughout the electrode.

In U.S. Pat. No. 5,543,239 issued to Virkar et al., an electrocatalystis incorporated into a electrode microstructure that is claimed toimprove the performance of a solid state ionic device by providing acatalyst and by improving electrical conductance. In this disclosure, aporous ionic conductor is applied to a dense electrolyte substrate. Anelectrocatalyst is then introduced into the porous matrix to produceelectrical continuity and a large three phase boundary line length. As aresult, the electrocatalyst is applied as a thin layer of smallparticles over the ionic conductor.

The electrode disclosed by Virkar et al., however, does not solve theproblem of electrode instability. It is known that vapor loss of noblemetals occurs at even medium SOFC operating temperatures. According tothe Thomson-Freundlich (Kelvin) equation, an important aspect of thevapor pressure difference across a curved surface is the increase invapor pressure at a point of high surface curvature. Thus, the smallerthe particle size, the higher the vapor pressure. This could causesignificant vapor loss for small noble metal particles at SOFC operatingtemperatures.

Furthermore, higher vapor pressure at the particle surface and lowervapor pressure at a neck between two particles makes smaller particlesmuch easier to be sintered. Thus, the microstructure of an electrodewith submicronic noble metal (<0.5 μm) particles is not stable at mediumto high SOFC operating temperatures, and especially when the electrodehandles high current.

Furthermore, a thin electronic conducting layer at the electrode willhave large ohmic resistance at the electrode which limits the currentcarrying capacity of the electrode. As shown in the current-voltagecurves of the Virkar et al. patent, the experimental current is limitedto 0.5 A/cm² for the Pt/YSZ and LSM/YSZ cathodes disclosed therein.

Therefore, there is a need in the art for a composite electrode whichmitigates the limitations of the prior art, allowing higher performancesolid state ionic devices and solid oxide fuel cells in particular.

SUMMARY OF THE INVENTION

The present invention is directed at an electrode having an improvedmicrostructure which achieves a high density of active electrochemicalreaction sites between the electrolyte and electrode and incorporateselectrocatalytic materials such as noble metals into the electrode in anintimate fashion. As well, the improved microstructure also improveslong-term structural stability of the cell by reducing the effects ofnoble metal catalyst sintering and vapor loss. The electrode may beincorporated into any solid state electrochemical devices such as oxygenpumps, membranes and sensors, solid state batteries or solid oxide fuelcells. The electrode of the present invention may be either a cathode oran anode.

Accordingly, in one aspect of the invention, the invention comprises anelectrode forming part of a solid state electrochemical device, saidelectrode bonded to a dense electrolyte layer and comprising a porousthree-dimensional solid phase structure comprising:

(a) an electrocatalytic phase comprising a plurality of electrocatalyticparticles;

(b) an ionic conducting phase comprising a plurality of ionic conductorparticles;

wherein said electrical conducting phase and ionic conducting phase areinterspersed and wherein the mean size of said noble metal particles issubstantially equal to or larger than the mean size of said ionicconducting particles.

The electrode of the present invention is formed by mixing ceramic ionconductor particles and noble metal electrocatalyst particles into acomposite electrode which is then applied to a dense electrolytesubstrate by screen printing or by similar well-known methods. Theresulting electrode microstructure is highly porous and includes verylong three-phase boundaries, direct ion conducting channels from thecatalytic sites to the electrolyte and direct electron conductingchannels through the electrode to the catalytic sites. Theelectrocatalyst particles are preferably comprised of a noble metal andare preferably larger than the ion conductor particles which results ina morphology where the ion conductor particles pin the boundaries of thenoble metal particles. The relatively large noble metal particle sizereduces vapor loss at elevated temperatures while grain boundary pinningreduces or prevents sintering or coalescing of the noble metalparticles.

In one embodiment, the ion conductor particles may comprise ceramicparticles which may preferably be yttrium stabilized zirconia and thenoble metal particles may comprise palladium. Those skilled in the artwill be aware of other materials which will function as ion conductingparticles and as electrocatalytic particles.

In one embodiment, the invention may comprise an electrode comprising(a) an electrode functional layer for use in a solid stateelectrochemical device, said layer comprising a porous three dimensionalstructure comprising linked particles of an electrocatalytic materialand linked particles of an ionic conductor wherein the median size ofthe electrocatalyst particles is approximately equal to or larger thanthe median size of the ion conducting particles; and (b) a long rangeconducting electrode layer that is applied on top of the functionallayer. In a planar SOFC, long range conductivity refers to horizontalconductivity between the ribs of the interconnect plate, rather than theshort range vertical conducting path through the ceramics. Theconducting layer may comprise electronically conductive metal oxidessuch as lanthanum cobaltate.

In another aspect, the invention comprises a solid state electrochemicaldevice comprising a porous anode, a dense electrolyte and a cathodecomprising a porous three-dimensional structure comprising linkedparticles of an electrocatalytic material and linked particles of anionic conductor wherein the mean or median size of the electrocatalystparticles is larger than the mean or median size of the ion conductingparticles. The solid state electrochemical device may be a solid oxidefuel cell.

In another aspect of the invention, the invention is a method of formingan electrode for use in a solid state electrochemical device having adense electrolyte layer comprising the steps of:

(a) mixing electrocatalytic particles with ion conducting particleswhere mean or median size of the electrocatalytic particles issubstantially equal to or larger than the mean or median size of the ionconducting particles; and

(b) creating a porous three-dimensional structure bonded to the denseelectrolyte layer, said structure comprising linked particles of thenoble metal particles and linked particles of the ionic conductor.

In one embodiment, a further conducting layer of metal oxides may beapplied but are not presintered to the electrode. The metal oxides maycomprise lanthanum cobalt oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of an exemplary embodimentwith reference to the accompanying simplified, diagrammatic,not-to-scale drawings. In the drawings:

FIG. 1 is a schematic representation of a cross-sectional view of acathode according to a preferred embodiment of the present invention.

FIG. 2 is a scanning electron micrograph of a cathode cross-sectionaccording to a preferred embodiment of the present invention.

FIG. 3 is a graph of the I-V characteristics of a fuel cell of thepresent invention.

FIG. 4 is a scanning electron micrograph (5000×) of a cathodecross-section of another embodiment of the invention.

FIG. 5 is a scanning electron micrograph of an alternative embodiment ofa cathode in cross-section.

FIG. 6 is a schematic depiction of current flow through the cathode andthe conducting electrode.

FIG. 7 is a graph of the I-V characteristic of one alternativeembodiment of a single fuel cell.

FIG. 8 is a graph of the I-V characteristic of one emboddiment of a 15fuel cell stack.

FIG. 9 shows the effects of altering the noble metal concentration onpower density, with tests conducted at different temperatures.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for a composite electrode for use with asolid oxide fuel cell and further provides for a method of making suchan electrode. When describing the present invention, all terms notspecifically defined herein have their common art-recognized meanings.

A. Definitions

As used herein, the term “about” refers to a range of values that is thestated value plus or minus 10%.

As used herein, the term “electrocatalyst” refers to a material which isboth electronically conducting and a catalyst for an electrode reaction.Electrocatalyst materials may include noble metals and certain metaloxides.

As used herein, the term “noble metal” refers to metals and alloys ofthe group comprising silver, gold, iridium, osmium, palladium,ruthenium, rhodium and platinum.

As used herein, the term “LC” or “lanthanum cobaltate” refers to LaCoO₃.

B. Description

As shown in FIG. 1, a porous composite electrode (10) is shown bonded toan electrolyte (12). The composite electrode is formed fromelectrocatalytic noble metal particles (14), and from ion conductingceramic particles (16) which are bonded intimately to the electrolyte(12). The ceramic particles combine to form ionic conducting paths (I)from the electrolyte (12) to the electrochemical active sites (18). Themetal phase forms electronic conducting paths (E) through the electrode(10) to the contact paste (not shown) and cathode electrical conductingstrip (not shown). The electrochemical active area coincides with thethree phase boundary (18) which extends along the common boundary of thegaseous pore phase, the ceramic phase (16) and the noble metal phase(14). It is generally believed that the electrode reaction substantiallytakes place at this boundary, where the three phases (gas, electronconductor and ion conductor) meet.

Thus, the composite electrode of the present invention may provide moreelectrode reaction sites to lower the overpotential loss. Furthermore,the presence of catalytic noble metals at the electrochemical activeareas (18) lowers the activation energy for the electrode reactions.

The ceramic ionic conducting phase in the composite electrode may be anyknown ion conductor such as yttria stabilized zirconia (YSZ). In apreferred embodiment, the ceramic phase is preferably the same materialas the electrolyte so that interface between the ceramic phase and theelectrolyte is chemically stable and there is a good thermal matchbetween the two materials.

The electrocatalytic phase may be any noble metal or noble metal alloy.These metals all have catalytic effect for the reduction of oxygen andare good electronic conductors. In the preferred embodiment, palladiumis mainly used because its coefficient of thermal expansion is similarto that of the YSZ which may be used as the electrolyte and in theceramic phase. Accordingly, the use of palladium and YSZ in thepreferred composite electrode of the present invention provide goodthermal stability even where the electrode is subjected to thermalcycling.

The relative proportion of the noble metal and ceramic ionic conductingphases may be varied. However, if the volume percentage of one phase islowered too far, continuous channels of that phase may not form when theelectrode is formed. It is preferable to have continuous ionicconducting channels, electronic conducting channels and porous channelsthroughout the composite electrode thickness.

The electronic conducting channels lowers the ohmic resistance of thecell. Electronic conductivity of the composite electrode may beincreased by increasing the particle size of the noble metals and byincreasing the volume percentage of the metal phase. However, increasingthe particle size decreases the catalytic effect of the electrocatalyst.Ionic conductivity may be increased by decreasing the particle size ofthe ceramic material and by increasing the volume percentage of theceramic phase. However, a longer three phase boundary is created byusing smaller particles of either the ceramic or metal phase.

As shown in the Figures, because the ceramic particles are preferablysmaller than the metal particles, the ceramic particles (16) partiallycover the noble metal particles (14). This reduction in surface area ofthe metal phase reduces vapor loss of the noble metal at elevatedoperating temperature. Moreover, the ceramic particles (16) tend toagglomerate between two adjoining metal particles (14), in an effectknown as grain boundary pinning, which prevents further sintering ofnoble metal particles. Thus, the morphology of the electrode, theelectrode/electrolyte interface and the three phase boundary is morestable.

In one embodiment illustrated in FIG. 1, the gas phase, the metal phaseand the ceramic phase are approximately equal in volume percent.However, the metal particles are approximately 5 to 10 times the size ofthe ceramic particles. The resulting microstructure is as shown in FIG.1 and FIG. 2. As is apparent, the ceramic particles form continuous ionconducting channels in the form of particle chains to the electrolytefrom the three phase boundary. The metal particles connect to formcontinuous electron conducting channels between the three phase boundaryand the cathode conducting layer. Finally, the high porosity of thestructure combined with the intertwining of the ion conducting channelsand the electron conducting channels creates a tremendously large threephase boundary.

A feature of the present invention is the relative size of the metalparticles compared to the ceramic particles. The metal particles shouldpreferably be larger than the ceramic particles and more preferablyabout 2 to 10 times larger. As a result of this size differential, theceramic particles tend to agglomerate on the metal particles incontinuous strings. In particular, the ceramic particles agglomeratealong the contact patches of adjoining metal particles. As referred toabove, this morphology not only increases the three phase boundary ofthe cathode but also reduces sintering of the metal particles andreduces evaporative loss of the metal.

An electrode according to the present invention may be applied to anelectrolyte/anode substrate according to well known suitable techniquessuch as screen printing, tape casting, slip casting, vapor deposition orthermal spraying. A preferred method is screen printing using a pasteformed from a suitable binder, a suitable solvent, the noble metalparticles and the ion conductor particles. The nature and use of thebinder and the solvent are well known to those skilled in the art.

In an alternative embodiment of the invention, a porous compositefunctional cathode layer (110) is shown bonded to an electrolyte (112)in FIGS. 4 and 5. The composite functional layer (110) is formed fromelectron conducting and catalytic noble metal particles (114), and fromion conducting ceramic particles (116) which are bonded intimately tothe electrolyte (112). Covering the functional layer (110) containingthe catalytic noble metal particles (114) and the ion conducting ceramicparticles (116) is a high electronically conductive layer (120). In oneembodiment, the electronically conductive layer (120) is made from LCmaterial. Other suitable materials for use in an oxidization environmentmay include LSM (LaSrMnO₃), or other electronically conducting metaloxides.

In one embodiment the functional layer (110) comprises noble metalparticles about 1 μm in diameter and is about 1-5 μm thick, and thusabout 1 to 5 particles thick. This provides a layer with good shortrange vertical electrical conductivity, since the probability of noblemetal particles providing an electrical conducting path between theelectrolyte and the LC layer (120) is greatly increased over the priorart, which features thicker layers and the same quantities of noblemetals. The ceramic particles are preferably smaller than the metalparticles and may be in the range of about 0.1 to 0.2 μm in diameter.

In one embodiment, the electrode layer (10) or the noble metalfunctional layer (110) is comprised of 50% electrocatalytic particlesand 50% ion conducting particles with abut 33% porosity by volume. Inother words, the electrode comprises ⅓ ion conducting particles, ⅓electrocatalytic particles, and ⅓ pore space by volume. This volumepercentage of the electrocatalyst may be varied between about 1.0% andabout 95% by volume of the solid portion of the electrode, andpreferably between about 20% to about 60%, depending upon the costtarget to be achieved, desired performance per cell, or other factors.The volume percentage of the electrode taken by pore space is preferablyabout 30% or ⅓, although the electrode porosity may be higher or lower.

Where the electrocatalyst is a noble metal, the volume percentage ofnoble metal may be between about 1% to about 50% to achieve a goodbalance between cost and performance. As shown in FIG. 9, there is noincrease in performance in cells having a noble metal content in thefunctional layer (110) higher than 50%, therefore the additional cost ofthe extra noble metal is not preferred. Where the highest performance isdesired, the noble metal content is preferably about 50%. Where abalance of performance and cost is desired, the noble metal content ispreferably about 5%. Although the cell performance decreases as thenoble metal content is decreased towards 1%, the loss in performance maybe offset by the reduced cost of manufacturing the cell.

In an embodiment where the noble metal layer is thin, less than about 5μm and when the concentration of noble metals is below about 30 vol %,vertical conductivity (as indicated by arrow V in FIG. 6) is much moreprobable than horizontal long range conductivity (H) because of thedistribution of the noble metal particles. There may not enough metalparticles to provide continuous long range (H) conducting paths to carrycurrent to the relatively widely spaced ribs (130) of the interconnectplate (132). To overcome this difficulty, an electrode (120) ofelectronically conducting material is applied over the functional layer(110). This conducting electrode (120) may preferably be in the order ofabout 15 to about 20 μm in thickness, but can vary from about 3 to about100 μm. The conducting electrode or “LC layer” (120) material ispreferably lanthanum cobaltate (LaCoO₃), which, in an oxidizingenvironment, has very good electronic conductivity properties althoughother suitable conductive materials may be used.

The LC layer is preferably not prefired prior to operation in the stackbecause it is preferred to avoid sintering of the LC layer. Oncesintered, the LC layer has a thermal expansion rate about twice that ofthe remaining components in the fuel cell, with the resultant sealingand bonding problems due to thermal expansion mismatch. Also, LC canchemically react with YSZ forming undesirable phases at the hightemperature encountered during sintering. For this reason, the LC layeris not sintered prior to use within the fuel cell stack.

The combination of the thinner noble metal functional layer (110) andthe long range electronic conducting LC layer (120) have produced a fuelcell that delivers improved performance over the prior art, and maydeliver power densities in the region of 1.2 W/cm² as shown in FIG. 9.

The following examples are intended to be illustrative of the claimedinvention but not limiting thereof.

EXAMPLE 1

This example discloses a method of making a Pd and YSZ composite cathodefor an anode supported solid oxide fuel cell. The resulting cathode isschematically illustrated in FIG. 1. A scanning electron micrograph of acathode made in accordance with this example is shown in FIG. 2.

A screen printable composite cathode paste was made up of equal volumesof well-dispersed Pd particles, 8 mole percent yttria stabilizedzirconia (8YSZ) in alpha-terpineol. Ethyl-cellulose binder was added inan effective amount. The Pd particle size ranged from 0.5 to 2 μm with amedian size of about 1 μm while the 8YSZ particle size ranged from 0.1to 0.2 μm with a median size of about 0.17 μm. The substrate (100 mm insquare) consisted of a fully dense 8YSZ electrolyte (10 μm thick) on aporous NiO-8YSZ anode (1 mm thick). The cathode paste was screen printedon the electrolyte side of the substrate. The foot prints were 90 mm insquare. The prints were oven dried at 60-80° C., then fired at 1300° C.in air for 2 hours. The thickness of the composite cathode after firingwas about 5-10 μm. The resulting solid phase was 50% vol Pd and 50% volYSZ with approximately 33% porosity.

A comparison of the resulting Pd/8YSZ cathode cell with a similar cellwith a common perovskite cathode (LSM) showed that the cell with Pd/8YSZcathode had much better performance. A 15-cell stack made from cellswith this composite cathode was tested at 750° C. and achieved a powerof 750 W with hydrogen/argon (50/50) mixture as the fuel. Currentinterrupt experiments showed that the improvement resulted from bothlower ohmic resistance at the cathode due tot he palladium conductivitynetwork and lower overpotential loss due to an increase in theelectrochemical active area (three-phase boundary) and catalytic activearea (palladium surface).

FIG. 3 illustrates the I-V characteristics of a single fuel cellincorporating this embodiment of an electrode operating at temperaturesvarying from 600° C. to 900° C.

EXAMPLE 2

This example discloses a Pd, YSZ, and LC composite cathode for an anodesupported solid oxide fuel cell and a method of making such a cathode. Ascanning electron micrograph of the resulting cathode is illustrated inFIG. 4.

A screen printable composite cathode functional layer paste was made upof suitable volumes of well dispersed Pd particles and 8YSZ in alphaterpineol to achieve a solid phase of 5% Pd/95% 8YSZ vol. Ethylcellulose binder was added in an effective amount. The Pd particle sizeranged from 0.5 to 2 μm with a median size of about 1 μm while the 8YSZparticle size ranged from 0.1 μm to 0.2 μm with a median size of about0.17 μm. The substrate (100 mm square) consisted of a fully dense 8YSZelectrolyte (10 μm thick) on a porous NiO-8YSZ anode (1 mm thick). Thecathode functional layer paste was screen printed on the electrolyteside of the substrate. The footprints were 90 mm square. The prints wereoven dried at 60-80° C., then fired at 1300° C. in air for 1 hour. Thethickness of the composite functional layer after firing was about 1-3μm. The LC layer was screen printed to a thickness of about 3 μm on topof the functional layer but was not sintered. Once the cell is atoperating temperature of 800° C. the LC powder bonded adequately to thefunctional layer.

FIG. 7 illustrates the I-V characteristic of a single fuel cellincorporating this embodiment of a cathode at operating temperaturesvarying from 600° C. to 900° C.

FIG. 8 illustrates the I-V performance of a stack of 15 fuel cellsincorporating this embodiment of a cathode.

EXAMPLE 3

A composite cathode was screen printed in a similar manner as Example 2above but to a thickness of about 10 μm. The LC layer was again screenprinted on top of the functional layer but to a thickness exceeding 30μm. A scanning electron micrograph showing the resulting cathode incross-section is shown in FIG. 5.

EXAMPLE 4

FIG. 9 illustrates the effect on power density (w/cm² at 0.7 v) ofvarying the proportion of palladium from 0% vol. to 95% vol. of thesolid phase. As may be seen, performance is maximized with 50% vol. Pd.However, significant performance is still achieved with Pd loading aslow as 5% vol.

As will be apparent to those skilled in the art, various modifications,adaptations and variations of the foregoing specific disclosure can bemade without departing from the scope of the invention claimed herein.

What is claimed is:
 1. A method of forming an electrode for use in asolid state electrochemical device having a dense electrolyte layercomprising the steps of: (a) mixing electrocatalytic particles with ionconducting particles where the mean or median size of theelectrocatalytic particles is substantially equal to or larger than themean or median size of the ion conducting particles and the ratio ofelectrocatalytic particles to ion conducting particles is about 65:35 orless by volume; and (b) creating a porous three-dimensional structurebonded to the dense electrolyte layer, said structure comprising linkedparticles of the electrocatalytic particles and linked particles of theionic conducting particles.
 2. The method of claim 1 wherein theelectrocatalytic particles, the ion conducting particles, a suitableorganic binder and a suitable solvent are mixed in appropriate volumesto form a paste which is then screen printed onto the dense electrolyte.3. The method of claim 1 wherein the electrocatalytic particles arecomprised of a noble metal.
 4. The method of claim 3 wherein the noblemetal comprises palladium.
 5. The method of claim 1 wherein the ionconducting particles are comprised of YSZ.
 6. The method of claim 1further comprising the steps of sintering the electrode and thenapplying a long range electrical conducting layer comprising a metaloxide.
 7. The method of claim 6 wherein the metal oxide compriseslanthanum cobaltate.
 8. The method of claim 1 wherein the volumepercentage of electrocatalytic particles in the solid phase of theelectrode is between about 1% and about 50%.
 9. The method of claim 1wherein the electrode is a cathode.
 10. A method of forming a cathodeforming part of a solid oxide fuel cell comprising a dense electrolytelayer and an anode layer, the method comprising the steps of: (a) mixingelectrocatalytic particles with ion conducting particles wherein themean size of the electrocatalytic particles is substantially equal to orlarger than the mean size of the ionic conducting particles; and (b)creating a porous three-dimensional structure bonded to the denseelectrolyte layer, said structure comprising linked particles ofelectrocatalytic particles and linked particles of the ionic conductor;wherein said structure is less than about 10 micrometers thick.
 11. Themethod of claim 10 wherein the cathode is formed less than about 5micrometers thick.
 12. The method of claim 11 wherein the cathode isformed less than about 3 micrometers thick.
 13. A method of forming anelectrode forming part of a solid state electrochemical device, saidelectrode bonded to a dense electrolyte layer, said method comprisingthe steps of: (a) mixing a plurality of electrocatalytic particles witha plurality of ionic conductor particles wherein the mean size of theelectrocatalytic particles is substantially equal to or larger than themean size of the ionic conducting particles; and (b) creating a porousthree-dimensional structure bonded to the dense electrolyte layer, saidstructure comprising linked particles of the electrocatalytic particlesand linked particles of the ionic conductor particles, and wherein thethree dimensional structure has a vertical dimension and a horizontaldimension and the electrocatalytic particles provide continuouselectronic conductivity in the vertical dimension but not in thehorizontal dimension.
 14. The method of claim 13 further comprising thestep of applying a horizontally electronic conducting layer comprisingan electronically conductive metal oxide.
 15. The method of claim 14wherein the metal oxide comprises lanthanum cobaltate oxide.